TVA's (tuned vibration absorbers) are conventionally fixed-tuned and are utilized to reduce unwanted vibration of an object structure at a particular disturbing frequency, e.g. a constant motor speed or reciprocating parts vibrating at a constant frequency.
A fixed-tuned TVA is typically a spring/mass resonator consisting of a vibratable mass attached indirectly to the object structure by a spring support or suspension means that allows the mass to vibrate at the disturbing frequency in response to excitation received from the object structure, in an interactive manner that reduces the vibration amplitude of the object structure by absorbing energy therefrom.
The spring material in the suspension means is characterized by a physical property known as spring modulus, sometimes referred to as spring constant or spring rate, defined as force/deflection distance, where the force can be regarded either as an applied deflecting force applied externally or as an opposite and equal restoring force exerted internally by the spring. A related physical property, compliance, defined as the displacement of a linear mechanical system under a unit force, is inversely related to spring modulus.
The spring modulus of the suspension means in combination with the mass determines a natural resonant frequency of vibration, which is typically tuned to be equal or close to the frequency of the disturbing vibration, hence the name TVA (tuned vibration absorber).
In a linear-motion type TVA the mass is constrained laterally so it can move only along a straight line path and is constrained longitudinally by the spring means, e.g. coil springs or flexures, so that under vibration it reciprocates only along a longitudinal path, typically coinciding with the central axis of a coaxial structure.
In a beam type TVA a support beam of spring material is cantilever-mounted at its base end onto the object structure and carries at the opposite end an enlarged head portion constituting the mass which can thus vibrate in an arcuate travel path, defined by flexure of the beam, transverse to a central axis of the beam, in a radial direction determined by the disturbing vibration in the object structure.
With a circular cross-sectional beam shape the vibration absorber will vibrate at the natural resonant frequency in any radial direction of excitation. A rectangular beam may have two predominant arcuate vibration paths perperdicular to each other, each with a different natural resonant frequency, while a thin wide flat beam, i.e. a leaf spring, the predominant arcuate vibration path traverses the leaf spring perpendicular to its flat surface.
Since the mass and mechanical spring modulus are practically constant, the resultant resonant frequency is also constant, hence, due to the fixed tuning, a simple spring-mass TVA is effective only at or near its constant natural resonant frequency; a shift in the disturbing frequency can render the fixed-tuned TVA useless or even detrimental by generating spurious vibrations at the TVA's resonant frequency.
Vibration control in equipment with variable speed, such as in air/land/water vehicles requires variable-tuned TVA's whose natural resonant frequency can be adjusted to accomodate variations in the disturbing frequency, e.g. engine speed variations in an aircraft, boat or land vehicles.
A mechanically tunable vibration absorber can be retuned to a different frequency mechanically, optionally assisted hydraulically or pneumatically, e.g. by altering the beam length or the amount of mass, e.g. pumping liquid in or out, however such mechanical modes of manipulation are generally impractical, especially under operating conditions, therefore there is widespread need for a more agile and convenient mode of frequency adjustment for real-time manual control of vibration frequency, and especially for automatic control. By far the most agile and convenient mode of variable tuning would be an ETVA (electrically-tunable vibration absorber), e.g. where the resonant frequency can be varied by adjusting the amplitude of an electric current.
It is known that an air gap set up between a pair of opposed magnetic pole pieces exerts an attracting force between the pole pieces; thus by making one of the pole pieces movable relative to the other a "magnetic spring" can be formed: this force can be conveniently varied by adjusting the current in an electromagnet from which the gap is magnetized, changing the magnetic flux density. Thus an ETVA (electrically tuned vibration absorber) car be implemented electromagnetically by supplementing or even supplanting the mechanical spring suspension by an electromagnetic spring system that enables; the mass to vibrate at a resonant frequency that can be varied as a function of D.C. current in the electromagnet.